Project Summary Huntington?s Disease (HD) is an autosomal dominant, neurodegenerative disease characterized by involuntary movement dysfunction, and cognitive and psychiatric symptoms, ultimately resulting in death. Although diagnostic testing is available for HD, there remains a critical need for an objective clinical marker to characterize both disease onset and progression. In order to develop effective therapeutics to support early intervention and prevent decline, we need to understand the early-stage biological changes in the living brain that occur at disease conversion. Imaging with positron emission tomography (PET) facilitates in vivo longitudinal measurements of molecular changes that manifest with evolving pathology. To date however, no PET ligands have been generated to predict disease conversion or aid prognosis in HD. Recent studies have shown that glial cell activation can be detected either at or just prior to the onset of symptoms in HD patients, evoking potential for the development of such a ligand. In addition, there is emerging evidence from neurodegenerative disease models that microglia, the brain?s resident immune cells, play an important role in some of the earliest pathological events, including synaptic loss. Our preliminary data showing increased cyclooxygenase-2 (COX-2) protein in both HD patient and HD mouse model brains (postmortem), suggest it holds promise as a novel clinical marker. Preclinical models exhibit elevated COX-2 during periods of microglia-mediated synaptic elimination, an event early in the pathology of many neurodegenerative diseases. In addition, we see COX-2 is increased specifically in the microglia of disease-affected regions in human HD post-mortem brain tissue. The only way to truly understand the role that COX-2 plays in HD is to examine its presence and dynamics in the human brain throughout the course of disease. Therefore, we propose to develop a selective radiotracer for in vivo PET imaging to study COX-2 dysregulation in the living brain, and carry out ex vivo mechanistic studies of COX-2 function in microglia. We will synthesize novel COX-2-selective ligands, optimized for affinity, target selectivity, high brain uptake, and amenability to radiolabeling with carbon-11 or fluorine-18. These ligands will undergo rigorous physiochemical and biochemical profiling, including assays that evaluate COX-2-isoform selectivity, and that predict blood-brain barrier penetration. Lead compounds will be radiolabeled and evaluated with in vitro autoradiography using human HD post-mortem brain tissue to evaluate specific and saturable binding. This will be followed by in vivo imaging in rodents with PET to evaluate brain uptake, radiotracer kinetics, and radiometabolites. In parallel, we will elucidate biochemical mechanisms of COX-2 in microglia in HD mouse models, and investigate the role COX-2 plays in altering microglial function. By the end of the grant project period, our team will have the ability to study COX-2 in the living brain with translational imaging tools, enabling improved understanding of HD pathophysiology and accelerating development of novel therapeutics.